If you are going to do only 1 SSD, put it in the data doubler.
You can find cheaper versions of this around amazon and ebay, and even get an external enclosure for your optical drive, turning it into a USB super drive, but the OWC version has the best ratings. It is not optimal for a spinning hard drive to be in the data doubler chassis, as it is not really designed to keep the spinning drive stable.
This laptop came with Mountain Lion, and it appears it's super easy to install the OG OS that the. I am the nostalgic type when it comes to operating systems, and I have an idea. I am also running bootcamp with Windows 10 too. I bought a 1TB SSD a while ago and it has been great so far. Mid-2012 13' MD101LL/A MacBookPro9,2 A1278 2554 Apple MacBook Pro 'Core i5' 2.5 13' Mid-2012 PIN: 204PIN Type: DDR3 Form Factor: SODIMM Technology: NON ECC Memory Ranking: 2RX8 Voltage: 1.35 Speed: PC3-12800, DDR3-1600 Kit Qty: 2 Total Location Capacity: 16GB Capacity Per Module: 8GB WARRANTY: Lifetime by CMS® Company Name. It is just a piece of plastic that holds the drive in place. I have a Mid 2012 MacBook Pro Retina that I love very much. The Data Doubler is the name OWC gave to its product to allow you to put a hard drive or ssd into the optical bay.